Jack Russell tries to bite postman

by Andrew
(UK)

My JRT is 20 months old and has recently started getting very aggressive when people come to the front door. She jumps up at the window barking and gives a very distinctive angry growl that seems to be reserved for these moments. If the door is ajar bcause the visitor needs to hand over a parcel or have documents signed, she wil try to get outside to bite them and get very angry with anyone who tries to restrain her. What's the reason for this and how dow e stop it? Any help would be greatly appreciated, thanks.